1. Field of the Invention
The present invention is related to a fluorescent lamp driving circuit; in particular, to a multi-lamp driving circuit with built-in MOSFET (Metal Oxide Semiconductor Field Effect Transistor).
2. Description of Related Art
A backlight module of a liquid crystal display uses high-frequency sinusoidal-wave power supply to supply power as the energy required for lighting the Cold Cathode Fluorescent Lamp (CCFL), therefore it is demanded to employ DC/AC inverters to achieve the purpose of energy conversion. An ordinary CCFL driving circuit uses a resonant module to convert a DC voltage into an AC voltage to drive a CCFL to light up. The driving voltage and the driving current of the CCFL are detected by the voltage and current detection circuits. A Pulse-Width-Modulation (PWM) controller receives the generated voltage detection signal and current detection signal for the purposes of stabilizing the illumination of the CCFL and circuit protection.
Due to the trend of large-scaled liquid crystal panel, the number of CCFLs in the backlight module needed to be driven increases accordingly, conventional driving circuit applying single PWM controller and single resonant module for driving single lamp may result in complexity in circuit design as well as increasing of production cost. In order to reduce cost of multi-lamp driving, it is common to apply one PWM controller to control the multi-lamp driving circuit, so as to reduce the number of components and simplify circuit design.
FIG. 1 shows a circuit diagram of a conventional multi-lamp driving circuit. The multi-lamp driving circuit comprises a PWM controller 100, a switch module SW, a resonant module, a multi-lamp module, a plurality of current detection modules 110, 130, and a plurality of voltage detection modules 120, 140, wherein the multi-lamp module consists of a plurality of lamps L1, L2, and the resonant module consists of a transformer T as well as resonant capacitors C1, C2. The switch module SW is connected to an input voltage Vin, and is utilized to control the energy transferred to the resonant module according to the control signals from the PWM controller 100. The secondary side windings of the transformer T are connected to the lamps L1, L2, respectively. The current detection modules 110, 130 are connected in series with the lamps L1, L2 respectively so as to generate current detection signals A1, A4 representing the magnitude of current passing through the lamps L1, L2 respectively, and also to generate lamp status signals A3, A6 representing the status of the lamps L1, L2, respectively. The voltage detection modules 120, 140 are connected in parallel with the lamp L1, L2 respectively so as to generate voltage detection signals A2, A5 representing the magnitude of voltage drop on the lamps L1, L2 respectively. The PWM controller 100 receives the aforementioned signals A1˜A6 to performs soft start and feedback control in order to control the power transferred by the switch module SW for stabilizing the illumination of the lamp and also for providing circuit protection upon the occurrence of abnormality in the circuit.
By using the above-described circuit, it is possible to use one PWM controller to control two lamps simultaneously, thus reducing the number of PWM controllers. But, the number of pins of the PWM controller and the required electronic components are still too many, as a result, how to effectively reduce the number of pins needed in the PWM controller and lessen the required electronic components so as to simplify the circuit design, is still a critical topic for present research and development of CCFL driving circuit.